Add :latest to docker image strings without a tag

Passes spec and CATs with latest cf-release and diego-release.

Has an API change because of the docker example so probably requires a cf-release bump. No submodules touched in this one that need to be bumped.

[#75755060]

Signed-off-by: Stewart Nickolas <nickolas@us.ibm.com>

/cc @onsi @hiremaga